1. Field of the Invention
The present invention relates to a blanket for an extended nip press. More particularly, the present invention relates to a blanket having a woven base reinforcement for use in an extended nip press of a papermaking machine.
2. Information Disclosure Statement
In the papermaking art, paper stock is ejected onto a moving fourdrinier wire such that water within the stock is drained therefrom leaving a formed web thereon. The formed web is subsequently guided between cooperating press members such that excess water within the formed web is pressed therefrom.
The cooperating press members usually include a pair of counter-rotating press rolls which define therebetween a pressing nip for pressing the formed web. However, more recently, extended nip presses have been employed for the removal of such water from the formed web.
An extended nip press essentially includes a rotatable backing roll and a cooperating elongate shoe which is movable towards the backing roll for defining therebetween an elongate pressing nip for pressing water from the formed web. A press blanket extends in a looped configuration through the elongate pressing nip such that the web is disposed between the blanket and the backing roll. Lubrication is provided between the shoe and the blanket such that the blanket slides in a machine direction relative to the shoe for supporting the web during movement of the web through the pressing section.
The aforementioned extended nip press has proved to be very successful in removing greater quantities of water from a formed web when compared with the aforementioned presses having counter-rotating press rolls. Primarily, the reason for such increased water removing capability resides in the increased residence time of the web within the elongate pressing nip.
More particularly, a first felt is disposed between the web and the blanket in order to absorb the water pressed from the web. However, the blanket must, of necessity, be formed of a non-porous material such that the aforementioned lubricant between the shoe and the blanket does not contaminate the first felt and the web. Consequently, it has been found necessary to vent the backing roll such that water pressed from the web is permitted to escape from the vicinity of the elongate pressing nip.
With the provision of a vented backing roll, it has been found necessary to provide a second press felt disposed between the web and the backing roll because otherwise the grooved, or otherwise vented backing roll, would mark the surface of the pressed web.
Accordingly, with the aforementioned double felted extended nip press arrangement, the surface characteristics of the resultant web are less than desirable for the production of writing or printing grade papers because both surfaces of the resultant web exhibit a relatively rough surface texture due to contact with the respective press felts.
Therefore, press blankets have been proposed which define a plurality of machine directional grooves or the like for assisting venting of the elongate press nip. Such vented blankets obviate the need for a vented backing roll and potentially permit the production of a web in which one side of the web directly contacts a smooth external surface of the backing roll, thereby generating a smooth printable surface to the resultant web.
Although the provision of a grooved blanket of the aforementioned type has already been proposed as, for example, in U.S. Pat. No. 4,908,103 to Cronin et al, many problems have been experienced in the practical production of such a blanket.
Amongst such problems is the problem of "barrelling" which is the tendency for the grooved channels defined by the blanket to compress under extended exposure to pressure. The collapse of such grooves greatly impedes the flow of water through these grooves and, in many cases, these "barrel-shaped" grooves completely fail to vent the press nip.
Also, there exists the tendency for a grooved blanket of the aforementioned type to delaminate under the extreme pressures envisaged in an extended nip environment. Such extended nip presses typically operate at pressures in the region of 6,000 pounds per linear inch and such pressures usually cause such grooved blankets to fracture and delaminate after a few hours of use thereof.
The present invention seeks to overcome the aforementioned problems by providing a blanket of polyurethane resin having a woven base reinforcement exhibiting anisotropic characteristics. More particularly, the woven base includes filaments disposed in a machine and a cross-machine direction. The cross-machine directional filaments have a greater stiffness than the filaments disposed in a machine direction such that the blanket is permitted to flex through the elongate pressing section and around the various guide rolls while the stiffness in a cross-machine direction inhibits barrelling of the grooves and delamination of the blanket.
Therefore, it is a primary object of the present invention to provide a blanket which overcomes the aforementioned inadequacies of the prior art blankets and which makes a considerable contribution to the paper pressing art.
Another object of the present invention is the provision of a blanket having anisotropic properties such that the modulus of elasticity of the cross-machine directional filaments of the base is greater than the modulus of elasticity of the machine directional filaments so that the blanket is permitted to flex during passage through an elongate pressing section while delamination of the blanket and barrelling of the grooves defined thereby is inhibited.
Another object of the present invention is the provision of a blanket having anisotropic characteristics and which is vented such that in use thereof, writing and printing grade papers are able to be produced thereby.
Another object of the present invention is the provision of a blanket having anisotropic characteristics, the blanket being formed by saturating a woven base and spirally winding such saturated base onto a mandrel and subsequently removing the resultant blanket from the mandrel after curing and grooving the same.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.